This invention relates generally to gas turbine engines, and more particularly, to methods and apparatus for controlling the operation of gas turbine engines.
Gas turbine engines typically include a compressor section, a combustor section, and at least one turbine section. The compressor compresses air, which is mixed with fuel and channeled to the combustor. The mixture is then ignited generating hot combustion gases. The combustion gases are channeled to the turbine which extracts energy from the combustion gases for powering the compressor, as well as producing useful work to power a load, such as an electrical generator, or to propel an aircraft in flight.
Gas turbine engines operate in many different operating conditions, and combustor performance facilitates engine operation over a wide range of engine operating conditions. More specifically, stable combustion facilitates preventing engine blowout and providing for engine rated thrust and/or power levels. Furthermore, for gas turbines operated with dry low nitrous oxide (DLN) techniques, combustion stability also facilitates controlling nitrous oxide (NOx) and carbon monoxide (CO) emissions.
At least some known DLN combustion systems utilize premixed fuel and air, and operate at lean fuel/air (F/A) ratios to facilitate reducing NOx emissions. Lean fuel/air ratios are defined as such if the ratio of fuel to air is below the stoichiometric ratio of fuel to air required for the fuel under consideration. However, a consequence of the lean, premixed operation is that the combustion system may operate near a lean blow out (LBO) boundary. Lean blow out or weak extinction is the point at which the mixture of fuel and air is no longer flammable; for premixed multi-nozzle systems, weak extinction can be defined as the point at which there is a significant drop in the combustion efficiency and/or complete extinction of the flame. The LBO boundary or constraint, if violated, may result in partial or complete blowout (i.e., loss of the combustion flame). Controlling operation near an LBO boundary is even more difficult in a can-annular combustion system where the F/A ratio may be varied from one combustor can to another combustor can. More specifically, the variable F/A ratio may cause some combustor cans to operate with leaner F/A ratio than others and during operations, if an LBO boundary is violated, the can-to-can variability may lead to loss of flame in one or several combustor cans. Depending on the control logic within the system, when one or several combustor cans experience a loss of flame, the gas turbine protection system may shut the engine down to protect the entire system. However, such unexpected shutdowns may damage machinery and may cause large replacement power expenses to be incurred.